This invention relates to a radio frequency antenna.
Many communications systems for vehicles require an antenna mounted to the vehicle. For example, vehicle radios have conventional antennas, either fixed or retractable. Vehicle cellular communication devices often have a vehicle-mounted antenna.
It is known to mount radio or cellular antennas in or to a glass window of a vehicle. Certain glass-mounted antennas have impedance matching networks that have a low DC resistance to allow diagnostic detection of whether the antenna is properly connected to the cellular transceiver.
Referring to FIG. 1, the diagram illustrates the input impedance of an example quarter wave antenna 10 of a known type for receiving cellular communications to a vehicle. The input impedance is represented by a resistive component 12 in series with an active element 14. The impedance ZA of the antenna is the sum of RA and XA, and is not suitable for provide a test signal to the antenna signal output because the antenna typically appears as an open circuit. Thus it is not possible for the transceiver to use a test signal to determine whether the antenna is connected to the transceiver.
FIG. 2 illustrates an example antenna 10 known in the art and includes an internal base 22 mounted on the interior side of a vehicle body member 27 (such as a planar or curved roof panel or glass windshield). On the exterior side of vehicle body member 27 is the external extending portion 20 of the antenna 10. The external extending portion 20 is connected to the internal base 22 by a mounting screw (not shown) so that the two sandwich the vehicle body member 27. An RF connector 24 for an antenna feed transmission cable (not shown) is connected through a radio frequency signal connection to the internal base 22. An access hole 26 is provided opposite the mounting screw from the connector 24. Within the internal base 22 are structures that are coupled to the antenna ground and signal output.
Advantageously, this invention provides an improved antenna suitable for mounting on a vehicle.
Advantageously, this invention provides an improved antenna that can be remotely monitored to ensure that it is correctly connected.
Advantageously, according to a preferred example, this invention provides a vehicle antenna comprising: an antenna structure having an input impedance, ZA, equal to a sum of a resistance and an active impedance; and a circuit element having high circuit element impedance at an operating frequency range of the antenna, wherein the circuit element is coupled in parallel with the input impedance.